1. Field of the Invention
This invention relates to a filter system and method for the removal of contaminants from a dielectric fluid. This invention further relates to a filter system for the removal of both water and other contaminates from a dielectric fluid using an electrostatic filter and a water-removal filter, and a method for using the filter system to supply a continuous supply of filtered dielectric fluid.
2. Description of the Prior Art
Prior art filters include filters for removing particulates from various dielectric fluids such as oils and hydraulic fluids by employing mechanical filtration. When mechanical filtration is employed to filter very fine particles, i.e., below about 3 microns, difficulties may be encountered because these relatively small particles may require large bulk and volume filters to avoid significant system pressure drops caused by the necessarily small openings in the filter media. Alternatives to mechanical filters for fine particle filtration of dielectric fluids include electrostatic filters where the fluid is passed through a number of perforated electrodes which are alternately charged with relatively high positive and negative direct current (D.C.) voltages. Porous filter material is placed between the electrodes for trapping the particulates and extends across the entire internal cross-sectional area of the filter.
It appears that the particulates when subjected to the electric fields created by the application of voltage to the electrodes are captured by one of two possible ways. The filter media itself may be charged, with the particulates being attracted to the filter media itself. More likely, however, the particulates are charged either positively or negatively depending on their composition and the oppositely charged particles will be attracted to each other and eventually floc together into clumps which will be large enough to be trapped in the filter media. When enough clumps form to effectively block the filter or produce an undesirable pressure drop, the filter media must be replaced.
Electrostatic filters of this type are shown in U.S. Pat. No. 5,332,485 issued Jul. 26, 1994, to Donald E. Thompson, U.S. Pat. No. 4,594,138 issued Jun. 10, 1986, to Donald E. Thompson, and U.S. Pat. No. 5,630,926 issued May 20, 1997, to Donald E. Thompson. These patents are hereby incorporated in their entirety by reference.
These patents disclose filters where the flow of the contaminated fluid through the filter is axial and utilize perforated electrodes separated by layers of filter media. The electrodes are alternately oppositely charged with the filtration process taking place by flowing the contaminated fluid upwardly through perforations in the electrodes and the filter media between the plates.
While filters of this type may be effective, they have certain drawbacks. The amount of surface area available for filtration is limited to the internal cross-sectional area of the inner chamber of the filter housing. Accordingly, if water or other contaminates reach a level sufficient to permit short circuits between the perforated electrodes, plug the filter media and the like in the first layer of filter media, the filter is rendered less effective, may require a much higher pressure and may require replacement of the filter cartridge. This is a very undesirable result since the bulk of the cartridge may remain relatively uncontaminated and could be used for a longer period were not the cross-sectional part of the media nearest the inlet overloaded with contamination. It is a further disadvantage that the diameter of such filters must be increased to increase the internal cross-sectional area. This can result in a very large diameter filter or a large number of filters when higher flow rate is desired.
Accordingly, a continuing effort has been directed to the development of improved electrostatic filters.